Oblivion is storyless, boring world, G rated game. If you actually like story, like to have a reason to do quest, and feel that G rated games are not for you, then you will like the Witcher.

Oblivion and Witcher are at two different ends of the western RPG spectrum. Yeah, Oblivion doesn't have much story, but it's free-form and heavily modable world where you can do pretty much what you want with a character of your own creation (which is why I've got over three hundred hours in that game).

Witcher and Witcher 2 are a lot more story focused, centered on one character with a defined skill set; it's world is more restricted and you're not modding it. That doesn't mean it isn't a lot of fun (especially since the story is kicking ass so far), but it's not going to appeal to a player that only likes Oblivion style RPGs. Personally, I like both styles (but I can't stand Japanese RPGs).